I have the 20M on order for TTL fill-flash. (Very hard to get in the UK for some reason!)

For manual flash the Yongnuo wireless trigger and receivers work well, my current set:

RF602 trigger and one receiver

560 iii flashgun (has a built in wireless receiver but will also fire directly from camera)

MR 58 macro ring flash (fires on receiver or directly from hotshoe)

All 3 of the Yongnuo items listed above had a combined cost (£120) of less than the 20M (£130), but I do want a small TTL solution for on-camera fill - hence the 20M.

I expect the 20M to work with the wireless receiver from the RF602 (I have tested it on Canon and Yongnuo flashes) so with the current set up I'll be able to have on manual both the 20 and the 560 or the ringflash and the 560, but additional receivers are £8 so I could even have all 3 going at once.

Of course with the Sony 600 you get TTL, but it costs £460 and only one light source!!